1. Field of the Invention
The present invention relates to a gate driver for driving the gate of a switching element.
2. Description of Related Art
A GaN device has great potential compared with an existing Si device, and therefore, is anticipated for practical use. A standard GaN FET is a normally-on device, and therefore, needs a negative power source.
On the other hand, a normally-off GaN FET is very difficult to produce. The normally-off GaN FET has a threshold voltage of about +1 V, which is very low compared with that of an existing Si MOSFET. This is a first problem related to the normally-off GaN FET.
The normally-off GaN FET has an uninsulated gate-source configuration, unlike the Si MOSFET having an insulated gate-source configuration. The normally-off GaN FET, therefore, has a diode characteristic to pass a large current when a large voltage is applied thereto. Due to this, the normally-off GaN FET easily breaks if a large voltage is applied to the gate thereof. This is a second problem related to the normally-off GaN FET.
Due to these problems, a gate driver for the Si MOSFET (IGBT, or Insulated Gate Bipolar Transistor) is not applicable as it is to the normally-off GaN FET. Namely, the normally-off GaN FET needs an exclusive gate driver.
In connection with the first problem, a turn-off time of the normally-off GaN FET will be shortened if a voltage sufficiently lower than the threshold voltage of about +1 V, preferably a negative voltage lower than 0 V, is applied to the gate of the normally-off GaN FET. For this, the normally-off GaN FET needs a negative power source. The negative power source, however, is undesirable although it is necessary for realizing a normally-off state.
In connection with the second problem, a turn-on time of the normally-off GaN FET will be shortened if a voltage sufficiently higher than the threshold voltage is applied to the gate of the normally-off GaN FET. More precisely, shortening the turn-on time needs an instantaneous large current and creating the large current is properly achievable with a higher voltage. It is unable, however, to apply a high voltage such as 10 V used for the Si MOSFET to the gate of the normally-off GaN FET because it breaks the normally-off GaN FET.
To solve both the first and second problems, a related art illustrated in FIGS. 1A to 1C arranges a parallel CR circuit including a capacitor C1 and a resistor R1 at a location where a gate resistor of a MOSFET driver is generally arranged.
A similar related art is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2010-51165.